The present proposal concerns two main lines of neuroanatomical research. One is directed to the identification of cells of origin of ascending spinal tracts involved in somesthesis in cats and monkeys and to ascertain whether or not, at least some of these neurons may give origin, by way of collateral branching, to fibers destined to two or more different supraspinal targets. This project will be based upon the use of light microscopical techniques and of the retrograde transport of horseradish peroxidase. This technique has been improved upon in the P.I.'s laboratory and will be used in conjunction with other labelling strategies (chromatolysis and, eventually, use of deactivated, tritiated HRP). The second main research project consists of an investigation aimed at identifying interneurons within the dorsal column nuclei of cats and at providing anatomical data for a model of the intrinsic circuitry of these nuclei. Several modifications of the Golgi method and experiments with the horseradish peroxidase technique at light and electron microscopical level, will be employed for the tentative identification of interneurons. Analysis of synaptic endings of intrinsic origin and reconstruction of their relationship with relay cells, interneurons and their processes will be carried out at the electron microscopical level on serial sections and with the aid of a newly described, improved Golgi-E.M. method.